The present invention relates to image forming apparatuses, such as a copier, a facsimile machine and a printer of electro-photographic system, particularly relates to an image forming apparatus for transferring a toner image on a photoreceptor onto an intermediate transfer member and concurrently transferring the toner image having a plurality colors superimposed on the intermediate transfer member onto a sheet by a secondary transfer section.
There has been provided a tandem system color image forming apparatus among color image forming apparatuses of an electro-photographic system to realize a high-speed requirements in a high-speed trend in recent years. The tandem system color image forming apparatus forms toner images onto a plurality of photoreceptors, superimposes the toner images onto the intermediate transfer belt and concurrently transfers the superimposed toner images of a plurality of colors onto a sheet by a secondary transfer section.
In the tandem system color image forming apparatus having a certain type of mechanical structural disposal, there is a case that the distance from the place where starting image formation with the first toner color, for example, starting image formation with a Yellow color to the secondary transfer section, is longer than the distance from a sheet feed tray to the secondary transfer section.
In case of this mechanical structural disposal, at the time when the sheet is fed from the sheet feed tray, image formation has already started. At that time, even though the image formation is stopped after detecting the sheet exhaustion of the sheet feed tray, since the image formation has already started, the toner carried on the intermediate transfer member is not transferred to the sheet and resides on the intermediate transfer member as a residual toner. Further, the residual toner on the intermediate transfer member was transferred onto a secondary transfer roller and the transferred toner was re-transferred onto the rear surface of sheets, which generates inconvenience, such as rear surface dirt.
If the number of sheet left in the sheet feed tray is accurately detected and the timing of sheet exhaustion is accurately determined, the occurrence of the residual toner can be avoided by stopping the image formation with that timing. However, accurately detecting the number of sheet left is practically difficult to achieve.
Unexamined Japanese Patent Application Publication No. 61-277536 disclosed a copier, which is arranged to start sheet feeding and image formation by delaying the sheet feeding timing and after confirming that sheet is not exhausted in case when the sheet left in the sheet tray becomes equal to less than a prescribed value as a countermeasure for those problems. Unexamined Japanese Patent Application Publication No. 2002-323839 discloses an image forming apparatus, which is arranged to prolong the sheet interval distance in case of continuous sheet feeding comparing with the normal situation so as to recognize the sheet exhaustion before starting exposure to the photoreceptor when having detected that the sheet left in the sheet tray becomes equal to less than a prescribed value.
However, according to the inventions of Unexamined Japanese Patent Application Publication Nos. 61-277536 and 2002-323839, since the distance between sheets widens when the sheet left in the tray is equal to less than a prescribed value, there is a problem that the productivity deteriorates after that until the sheet left becomes zero.